Brushless electrical machines such as, for example, synchronous motors can have, in addition to a stator winding, a field winding arranged on the rotor. The field winding is supplied with electrical energy to generate an excitation magnetic field. The electrical energy is provided to the field winding by virtue of transformer coupling between one or more primary coils on the stator and one or more secondary coils on the rotor. The field-circuit transformer formed from primary coils and secondary coils can be arranged in offset fashion, for example axially, on an axis of a rotary electrical machine. In other cases, the stator winding can be used as the primary coil of the field-circuit transformer.
The field-circuit transformer includes a stator field winding as primary coil and a rotor field winding as secondary coil. The rotor field winding can be connected to the field winding via a diode rectifier bridge. As a result, the field winding is provided with a direct electrical current, the field current, which generates the excitation magnetic field.
For defined operation of such an electrical machine, for example for setting a determined torque, however, the field current through the field winding can be determined and regulated to determine the intensity of the magnetic field brought about by the field winding. Since the electrical machine does not have a tap for currents on the rotor side and does not permit reliable current measurement in any other way, the field current is estimated by measurements of electrical variables on the stator side.
Previous estimation methods have been based on a measurement of two phase currents and of two phase-to-phase output voltages of an actuating element with a high time resolution, which is converted in a complex manner. The output voltage of the actuating element is additionally a non-sinusoidal voltage, as a result of which the computation complexity in the evaluation for the determination of the field current can be high.